Mixing between a reference signal and a data signal is often necessary to extract information about an optical device or network. A probe or measurement signal and a reference signal originating from the same source typically mix or interfere, resulting in optical interference “fringes.” A positive fringe occurs when the light is in phase and constructively combines (interferes) to a greater intensity, and a negative fringe occurs when the light is 180 degrees out of phase and destructively combines (interferes) to cancel out the light. The fringe intensities can be detected and used to assess information about the device being probed. In interferometric sensing, a reference signal is mixed with a reflected probe signal whose phase and/or amplitude is modified by a parameter to be measured. The mixing produces an interference signal, and the amplitude of the interference signal depends on how efficiently the two optical signals mix.
Optical Frequency Domain Reflectometry (OFDR) has been used for many years to measure the time-response of optical systems. Most optical time-domain reflectometry systems are simple pulse-echo type systems where a source emits a short pulse that propagates down the fiber and scatters off of discontinuities. This scattered light returns back as echoes that are detected as a function of time. The intensity of the detected light is then plotted as a function of time, which can be converted to distance if the speed of light in the medium is known. OFDR may be used to provide data related to one or more optical characteristics (e.g., backscatter, dispersion, etc.) of a fiber or fiber optic device that is part of a fiber over relatively short fiber distances, e.g., less than several hundred meters, but with relatively high “spatial” resolutions, e.g., centimeters and less. High spatial resolution is valuable for many reasons. For example, it allows more precise location and/or determination of optical characteristic of “events” like fiber flaws, cracks, strains, temperature changes, etc. and devices like couplers, splitters, etc. High resolution also allows performing such operations with a level of precision that distinguishes between events or devices located close together. Without that high resolution, measurements for closely located events or devices cannot be made on an individual event or device level.
OFDR sweeps a laser through a continuum of frequencies and records the interference fringes as a function of the laser frequency. These fringes are then Fourier transformed to produce time-domain information. Because interference is involved, it is widely believed that the range of the OFDR measurement is restricted by the coherence length of the laser used as the source. The coherence length of a laser is the distance along the optical fiber where the light maintains a specified degree of coherence. Coherence is the attribute a wave whose relative phase is predictable during a resolving time T. If the wave is separated into two paths and then brought back together after some time, T, the waves will interfere and produce predictable fringes if T is less that the coherence time of the source, and random unpredictable fringes if T is greater than the coherence time of the source. This decrease in predictability is gradual as the delay difference passes through T, but once the random part of the phase exceeds 180 degrees, no real correlation will be discernable. OFDR is based on an assumption that the optical device being measured is within the coherence length of the laser used.
The coherence length (usually measured in units of nanoseconds of optical delay) is roughly equal to the inverse of the laser linewidth, i.e., the amount of frequency spectrum that the laser light occupies across its center frequency. OFDR instruments are commercially available which use an external cavity, narrow-linewidth tunable laser (e.g., 100 KHz) to obtain resolutions on the order of tens of microns over 10 s to 100 s of meters of length. External cavity, narrow-linewidth tunable lasers have long coherence lengths. Unfortunately, external cavity tunable lasers are very expensive. In general, high resolution is harder to achieve over longer fiber distances, and this is especially true for inexpensive lasers that typically have shorter coherence lengths.
Distributed feed-back (DEB) lasers are inexpensive when compared with narrow-linewidth tunable lasers but have wider linewidths, and thus, short coherence lengths. They are also rugged and readily available in a range of wavelengths and packages. Lasers currently used in CD and DVD players are even cheaper and have linewidth characteristics similar to the DFB lasers. Although the CD lasers are not commonly fiber-coupled (a disadvantage), they generate light that can be detected by silicon detectors which are much less expensive than InGaAs (Indium Gallium Arsenide) detectors. It would be desirable to be able to use such inexpensive lasers and detectors to achieve spatial resolutions on the order of 1 mm over longer fiber distances, e.g., hundreds or thousands of meters. Technology with this capability would be valuable and applicable to a wide range of applications.
While DFB and CD type lasers are inexpensive and readily available, they have relatively wide linewidths, e.g., on the order of 10 MHz, compared to external cavity, narrow-linewidth tunable lasers. Because of this, it has been assumed that they could only be used to perform OFDR-type measurements over ranges of less than 100 ns in delay, or less than a 10 meter length in reflection over an optical fiber. If the coherence time of a laser is 100 ns, then after about 100 nanoseconds the phase of the laser light propagating along the fiber, (based upon an initial measurement of the phase), changes and can not be predicted with any certainty. It is this random nature of the phase beyond the 100 ns time period, i.e., beyond the coherence length of this particular type of laser, that led to an assumption that such a laser cannot be used for interferometry when interferometer paths differ by more than 100 ns, i.e., when the length of the reflection path exceeds 10 meters. Since many OFDR applications require ranges greater than 100 nm (corresponding to a coherence time delay longer than 1000 ns), the casual observer naturally assumes that these lasers could not be used in these applications. The inventors discovered that this assumption was wrong.